Sulfidic ores, prevelant in the Western United States, contain amounts of copper, molybdenum, lead, iron and other elements and are mined for these metals. Separation or refining operations for this type of ore are commonly achieved by physically grinding or pulverizing the crude ore, followed by separation of copper, molybdenum and other metals through differential flotation methods. This invention applies to various chemical techniques used in differential flotation.
Through the use of differential flotation, sulfidic ores, which contain small amounts of copper sulfide and molybdenum disulfide, may be separated into fractions or concentrates which ultimately contain 90% or more copper sulfide by weight in a copper concentrate or up to 90% or more molybdenum disulfide (MoS.sub.2) by weight in molybdenum concentrate. These methods of differential flotation separation are generally well known and are in widespread commercial use.
Various differential flotation techniques have been used to separate and concentrate minerals, and many chemical compositions have been employed for this purpose. In some froth flotation techniques, the concentrate of a preferred metal salt floats to the surface of a flotation vessel while the non-preferred salts are suppressed and discarded. Other techniques concentrate one preferred metal salt by flotation, while another preferred metal salt is suppressed and later recovered from the tailings. In primary copper recovery, for example, a molybdenum suppressant, such as dextrin, can be added to a flotation cell to cause the molybdenum to sink while allowing the copper to float. The tailings containing suppressed material can then be added to another flotation cell for flotation (and recovery) of molybdenum. In an alternate process, a copper suppressant, such as Nokes Reagent, or alternatively sodium hydrosulfide, can be added to the primary flotation cell and the tailings added to a secondary cell for recovery of copper. This invention is primarily concerned, however, with the first approach.
Since many minerals occur as sulfides in nature, methods for separating metal sulfides from their environment are well known. For instance, one commercial process grinds the molybdenum ore to a fine sand and disperses the ground ore in water. The ore slurry is then pumped to a flotation cell where it is mixed with a lightweight oil. Air bubbles passing through the flotation cell carry the molybdenum particles to the surface in the form of molybdenum concentrate which is collected for further processing.
Through the use of certain chemicals in the final stages of flotation processing, the physical properties of the metallic sulfides can be altered permitting fine particles of the selected metallic sulfide to be further concentrated in the processing foam, or to otherwise float and be removed, while other metallic sulfides are relatively unaffected and remain dispersed or suppressed in the aqueous phase. Inorganic chemicals which affect the physical properties of sulfidic ores in this way and allow their separation include aqueous solutions of phosphorus, sulfur, arsenic, and antimony compounds, to name but a few. Organic chemicals which are useful in sulfidic ore flotation include hydrocarbon oils, xanthates, alcohols, and silicones, as well as various surfactants and wetting agents. It is believed that chemicals which function to suppress flotation of metal salts, or metal sulfides in the case of sulfidic ores, do so by altering the surface characteristics of one or more of the metal salts (sulfides) so that they remain in suspension, while allowing a selected metal salt (sulfide) to float to the surface.
In the case of molybdenum, the most important domestic source of this mineral is molybdenite ore which contains molybdenum in the form of molybdenum disulfide (MoS.sub.2), as well as sulfides of copper, lead, and iron. Suppression of lead sulfide (or copper sulfide) can be accomplished by the addition of a suppressant solution to the molybdenum concentrate, commonly known as Nokes Reagent, which is well known for this purpose. Typically, Nokes Reagent is prepared by reacting P.sub.4 S.sub.10 in an aqueous solution of sodium hydroxide at a molar ratio of about 1:14, respectively.
See, for example, U.S. Pat. No. 2,492,936, issued Dec. 27, 1949, to Nokes et al., which describes the preparation of Nokes Reagent in wet or dry form, and its use as a depressant for the froth flotation of molybdenum. This patent discloses the preparation of a dry product by suitable means such as spray drying. Where economics are favorable, the dry product can be prepared at a central location and shipped to the mining site for convenient use. Various other flotation methods and chemicals are also described in U.S. Pat. No. 2,811,255, issued Oct. 29, 1957, to Nokes et al., U.S. Pat. No. 2,608,298, issued Aug. 26, 1952 to Arbiter et al., and U.S. Pat. No. 3,785,488, issued Jan. 15, 1974 to Werneke.
The preparation of such Nokes Reagent solutions at the mining or refining site is frequently associated with numerous problems and difficulties, such as the evolution of poisonous gases, e.g. hydrogen sulfide. In addition, mixtures of P.sub.4 S.sub.10 and sodium hydroxide have been known to react with explosive violence if the reaction conditions are not properly or carefully controlled. Thus, safety in handling, storage and use as well as difficulty in preparation, continues to be a major problem of great concern throughout the industry, and is a major factor in the search for alternate technology.
The suppression of copper sulfide, and to some extent, iron sulfide, is generally accomplished by the addition of sodium cyanide to the molybdenum concentrate either concurrently with the Nokes Reagent or in a separate processing step. However, sodium cyanide is also hazardous to use and must be handled with extreme care. Further, the effluent resulting from the use of sodium cyanide in froth flotation is an environmental contaminate and government regulations require treating such effluents with an oxidizing agent, such as chlorine dioxide, which increases the cost of the flotation process.
Accordingly, it is an object of the present invention to provide a differential flotation reagent for molybdenum ore which is economical, convenient to use, and results in a cleaner flotation process due to the improved suppression of metallic impurities.
Another object of this invention is to provide a differential flotation reagent which will permit elimination of sodium cyanide in the flotation process without substantial loss of performance.